ci: link Docker images to Gitea repository via OCI source label

docker/metadata-action uses github.* context vars which are empty in
Gitea Actions. Explicitly set org.opencontainers.image.source using
gitea.server_url and gitea.repository so the container registry links
each image back to this repository.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/docker-build-backend.yml

@@ -48,6 +48,8 @@ jobs:
         uses: docker/metadata-action@v5
         with:
           images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          labels: |
+            org.opencontainers.image.source=${{ gitea.server_url }}/${{ gitea.repository }}
           tags: |
             type=raw,value=latest,enable={{is_default_branch}}
             type=ref,event=branch

.gitea/workflows/docker-build-buttplug.yml

@@ -48,6 +48,8 @@ jobs:
         uses: docker/metadata-action@v5
         with:
           images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          labels: |
+            org.opencontainers.image.source=${{ gitea.server_url }}/${{ gitea.repository }}
           tags: |
             type=raw,value=latest,enable={{is_default_branch}}
             type=ref,event=branch

.gitea/workflows/docker-build-frontend.yml

@@ -48,6 +48,8 @@ jobs:
         uses: docker/metadata-action@v5
         with:
           images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          labels: |
+            org.opencontainers.image.source=${{ gitea.server_url }}/${{ gitea.repository }}
           tags: |
             type=raw,value=latest,enable={{is_default_branch}}
             type=ref,event=branch